Method for improving tilt stability in a motor vehicle

ABSTRACT

A method for improving side roll, initially based on a conceivably unfavorable motor vehicle load to indicate a stability-critical transversal acceleration or related variable when the vehicle begins to travel. By observing the motor vehicle during travel, information on real mass distribution can be obtained. Whenever there is a danger of tilting during cornering, braking occurs in at least the front wheel that is located towards the outside of the bend, resulting in a reduction of lateral forces and transversal acceleration. An additional active motor vehicle suspension can also be provided.

TECHNICAL FIELD

The present invention relates to a method for improving the lateral tiltstability of a motor vehicle with at least two axles and two tracks.

BACKGROUND OF THE INVENTION

In his book “Fundamentals of Vehicle Dynamics”, Society of AutomotiveEngineers, Inc., Warrendale 1992, Chapter 9, p. 309-333, T. D. Gillespiedescribes different models for roll-over accidents. The conditions forexisting tilting risks are calculated beginning with a quasi-stationarymodel for a rigid motor vehicle and a quasi-stationary model for aresilient motor vehicle up to dynamic models and taking intoconsideration the natural roll frequency.

When the book was published, it was already known that lorries, trucks,buses, minibuses and off-road vehicles in case of cornering with a largeroll movement present a tilting risk due to an elevated center ofgravity and/or small track widths, but only recently resulted, that alsopassenger cars—particularly in case of sinusoidal steering movements—maybuild up oscillations which increase to such an extent that they tilt.Such a danger of tilting is increased considerably by inappropriatelyloading the vehicle, i.e., only on one side or on the vehicle roof,because the position of the mass center of gravity of the motor vehicleis displaced upwardly or to one side.

DE-A 197 46 889 discloses a system for increasing the lateral stabilityin case of cornering which is provided with a device for detecting theinclination. Said device either measures the level difference betweenthe right and the left side of the vehicle or the transversalacceleration of the vehicle in order to detect the roll angle betweenthe vehicle level and the road level. If said device recognizes atilting risk, braking the front wheel that is located towards theoutside of the bend causes a correcting yawing moment.

As already described above, the admissible transversal acceleration aswell as the admissible roll angle depend on the position, in particularthe level of the center of gravity of the motor vehicle.

It is thus the object of the present invention to provide a methodreacting appropriately to the danger of tilting even in consideration ofan unfavorable load.

This object is achieved by not considering the center of gravity of theempty motor vehicle at the beginning of the travel, but the mostunfavorable center of gravity of the motor vehicle taking intoconsideration the admissible vehicle load when a stability threshold iscalculated. The stability threshold can be a transversal acceleration ora value correlating with this, for example a yawing rate. But thedecision to use the transversal acceleration instead of the yawing ratehas the advantage that a metering device for the transversalacceleration which is installed in the vehicle automatically takes intoconsideration lateral road inclinations, e.g. in super-elevated curves.

The mass distribution existing in the vehicle at the beginning of thetravel will not change very much during travel: the passengers mightchange their seats and the tank level of the vehicle falls. But theseare only small and/or slow changes in comparison to the vehicle mass.During travel the sensor technology of the vehicle can make furtherobservations concerning the acceleration and deceleration behavior whichserve as a basis for a more precise calculation of the real position ofvehicle's center of gravity.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of a four-wheel motor vehicle frombehind when driving in a left-hand bend.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following variables are shown in the drawing:

G center of gravity of the motor vehicle 1 h_(G) level of the center ofgravity M R rotational center of a roll movement of the vehicle h_(R)level of the rotational center R s track width of the vehicle 1 φ rollangle F_(G) gravitational force of the vehicle 1 F_(yi) lateral force onthe wheel that is located towards the inside of the bend 2 F_(zi)vertical force on the wheel that is located towards the inside of thebend 2 F_(yo) lateral force on the wheel that is located towards theoutside of the bend 3 F_(zo) vertical force on the wheel that is locatedtowards the inside of the bend 3

The following considerations are made without restricting the universalvalidity for the vehicle 1 when driving in a left-hand bend. In order tobe applicable in the same degree to a right-hand bend, the terms of thefollowing equations have to be provided with signs, which has beenomitted for reasons of clarity.

The wheel 2 located towards the inside of the bend begins to lift offthe ground if the lever moments acting on the center of gravity aroundthe wheel tread of wheel 3 located towards the outside of the bend 3,which is in this case considered as a point, counterbalance each other.Counter-clockwise this means for a small φ and m as vehicle mass:

m*g*[s/s−φ*(h _(M) −h _(R))

with g=9.81 m/s².

Clockwise the virtual centrifugal force is −Fy=−a_(y)*m causing thelever moment:

−m*a _(ycrit) *h _(M)

Equating the lever moments, substitution of φ by means of R_(φ), thedimensionless roll rate φ*g/a_(y) indicating the change of the rollangle φ (in radius) together with the transversal acceleration (inmultiples of g) and solution for a_(ycrit) results in the followingcondition for the critical transversal acceleration which causes thewheel located towards the inside of the bend to lift off:${aykrit} = {\frac{g*s}{2h_{M}}*\frac{1}{1 + {R_{f}\left( {1 - {h_{R}/h_{M}}} \right)}}}$

The critical transversal acceleration is thus the smaller the higher thecenter of gravity is lying over the roll rotation center. Thus theposition of the center of gravity is important for statements on thelateral stability.

This simple calculation of the critical transversal acceleration is onlyvalid in quasi-stationary cases. In case of dynamic steering maneuverswith excursions similar to vibrations the vehicle may build uposcillations. Therefore a stabilizing system should have its entrythreshold value a_(yon) already in case of smaller transversalaccelerations:

a _(yon) =d*a _(ycrit)

with 0<d<1.

The exit threshold from a stabilizing control is defined analogously:

a _(yoff) =e*a _(ycrit)

with 0<e<d.

An additional variable for recognizing the danger of tilting may also bethe change, i.e. the time derivative å_(y) of the transversalacceleration a_(y) by which the build-up of oscillations may berecognized, in particular in case of maneuvers with changing dynamics.Exceeding a threshold value å_(yon) which depends on the real and thecritical transversal acceleration can be introduced as an AND-conditionfor taking stabilizing measures:

a _(y) >a _(yon)

and

å_(y)>å_(yon)

Or a general consideration may be made including a stability conditionaccording to:

i*a _(y) +j*å _(y) <a _(ycrit)

with i, j being parameters specific to the vehicle that have beendefined empirically, i being dimensionless and j having the dimension ofa time. As soon as

i*a _(y) +j*å _(y) >a _(ycrit)

is valid, a stabilizing measure is taken. Certainly also a square formor another form may be chosen instead of a linear relationship. The mostadequate relationship will be defined by a series of tests, ifnecessary.

According to the chosen entry condition the following exit condition isvalid:

a _(y) <a _(yoff)

and

å_(y)<å_(yoff)

whereby

å_(yoff) =f*å _(yon)

and 0<f<1; or

i*a _(y) +j*å _(y) <k*a _(ycrit)

with 0<k<1;

or a more complex criterion.

A high transversal acceleration is given if large lateral forces aretransmitted between tire and roadway. The friction in the tread contactarea, i.e. in the contact surface between roadway and tire defines theentity of the transmittable horizontal force. The vector sum resultingfrom longitudinal force and lateral force cannot exceed the maximumforce defined by the friction coefficient. A control intervention may becarried out which increases the longitudinal force by means of braking,thus reducing not only the maximum transmittable lateral force, but alsoa dangerous transversal acceleration. The motor vehicle will then adoptan understeering behavior and follow a bigger curve radius.

A stabilizing control intervention comprising an active, i.e. automaticbrake actuation, in any. case will brake the front wheel located towardsthe outside of the bend. On the one hand, the vehicle side locatedtowards the outside of the bend is more stressed when cornering, whilethe front axle is more stressed in case of braking. Thus the front wheellocated towards the outside of the bend can transmit considerably higherforces between tire and roadway than the other wheels so that one canexpect the biggest effect in this case. On the other hand the forcevector of the longitudinal force built up on the front wheel locatedtowards the outside of the bend bypasses the center of gravity of themotor vehicle on the side located towards the outside of the bend thussupporting stability. A similar supporting effect can be observed on therear wheel located towards the outside of the bend. Caution is advised,however, if the vehicle is not equipped with an anti-blocking system,because—as is generally known—the vehicle tail may be caused to swerve,if the outer rear wheel that is more stressed blocks before the frontwheels. The longitudinal force vector of the front wheel located towardsthe inner side of the bend only shows to the side of the center ofgravity that is located towards the outside of the bend, if the steeringwheel is turned extremely so that the longitudinal force normally actsagainst the stabilization, which however is only of small importance dueto the lateral force which is reduced in turn. It is of no advantage tothe lateral stabilization to brake the rear wheel located towards theinside of the bend since this wheel is stressed only to a small degree,thus playing only a minimum role with regard to the lateral forcetransmission, additionally shows a longitudinal force vector with anunfavorable course and would favor the swerving tendency in case ofblocking.

In principle a total loss of the side guidance should not be causedsince in this case the vehicle simply slips off the road which, ofcourse, is no desirable alternative to the tilting of the vehicle.

If a critical transversal acceleration is recognized during a brakingoperation induced by the driver, the existing brake forces have to beredistributed taking into consideration the desired lateral forcereduction, whereby the total braking moment must not be reduced. As longas the wheels remain on the ascending section of the known μ(ë)-curve,an increase of the braking power is uncritical. When reaching themaximum value it has to be considered, however, that a further increaseof the brake pressure causes a loss of the brake moment, even if it issmall.

For the above reasons it is recommended to have the brakes reduce thelateral forces only on the linear ascending section of the μ(ë)-curvebelow saturation, i.e. in the so-called partial braking area.

Additionally, an active vehicle suspension may be foreseen whichcompensates the roll angle at least partially by lifting the side of thevehicle located towards the outside of the bend. Such systems have beendeveloped, e.g. for trucks and busses.

At the beginning of the travel a value is assumed as stability thresholdfor the transversal acceleration which guarantees that tilting can mostprobably be prevented, if physically possible, by means of a controlintervention and with any load that is legally admissible. In the courseof the travel conclusions may be drawn with regard to the location ofthe center of gravity by observing the wheel sensor signals whichpossibly permit increasing said threshold.

Provided that the driving torque generated by the driving motor duringacceleration is known, the drive power, i.e. the longitudinal forcebetween tire and roadway, can be calculated. The wheel sensorspreferably of non driven wheels detect the achieved vehicle speed fromwhich the vehicle acceleration can be derived by means of timederivative. The vehicle mass is the result of the acceleration dividedby the driving power. The known vehicle-specific mass of all parts whichare not spring-loaded is subtracted in order to consider the mass mmoved in case of a roll or pitch movement.

Similar observations are certainly possible also during the brakingoperation with regard to the vehicle deceleration, whereby a definedbrake power is assigned to a brake pressure.

The deceleration of a motor vehicle can also be used to infer theexisting mass shift, i.e. the pitch angle of the vehicle, from slipdifferences on the front and rear axles for given decelerations. Withknown elasticity values in the vehicle suspension and a knownspring-loaded mass m the level h_(m) of the center of gravity can becalculated on the basis of the pitch lever moment. In case of vehicleswith four-wheel drive, the slip differences on front and rear axle canalso be established on the basis of the acceleration.

The determined level of the center of gravity can then be inserted intothe equation for calculating the critical transversal accelerationa_(ycrit) in order to modify with this the entry threshold value a_(yon)or a more complex entry condition.

This method of determinating the center of gravity can be used eitherimmediately at the beginning of the travel during the first accelerationand brake maneuvers, repeated after certain time intervals or duringeach suitable brake operation and/or acceleration (preferably withoutturning the steering wheel).

If the vehicle is provided with a system for controlling the yawingmoment, said system should be modified in some way. A yawing momentcontrol regulates the yawing rate of a vehicle with regard to a nominalvalue. Said nominal value is usually limited to a physically sensiblevalue. But the physical considerations in general take into account onlythe friction coefficient conditions of the road surface: in case of ahigh friction coefficient the maximum nominal yawing rate to be fixed ishigher than in case of a low friction coefficient. In case of a highfriction coefficient, however, the tilting danger is higher due to largetransmittable lateral forces. The nominal yawing rate should thus belimited in consideration of the critical transversal acceleration and/orthe entry threshold for the lateral stability control. This is inparticular critical in case of systems which counteract not only to anoversteering tendency, but also to an excessive understeering tendency.While trying to prevent an oversteering in principle reduces also atilting danger, trying to prevent an understeering can cause or supporta tilting danger. Therefore in case of doubt, the increase of thelateral tilting stability should have priority over avoiding theundersteering.

A brake system which realizes the method of side stabilization accordingto the present invention must provide the possibility of brakingactively the individual wheels. That means, that at least on the singlefront wheels the brake has to be actuated without intervention of thedriver. This condition is given e.g. in case of motor vehicles withfront wheel drive and drive slip control. Small modifications withregard to the valve arrangement permit the active braking of each of thewheels. But also vehicles provided with a system for controlling theyawing moment by means of a brake intervention, are equipped for activebraking, usually individually for each wheel. Anti-blocking systems, onthe other hand, normally depend on the brake pedal in order to build upbrake pressure.. Said systems can, for example, be equipped with anactive brake power booster or a self-priming pump linked to the brakefluid tank, and a stop valve in the brake conduit. It may also berecommended to ensure a control of the individual wheels of the rearaxle brakes by additional valves, if necessary.

What is claimed is:
 1. Method for increasing the lateral tilt stabilityof a vehicle with at least two axles and at least two tracks in whichmeasures are taken for preventing the side roll, comprising the stepsof: A) comparing a variable correlating with the transversalacceleration of the vehicle against a first threshold value wherein thethreshold value represents, at least at the beginning of the travel, thelowest value which leads to tilting risk in case of any admissible loadof the vehicle, B) introducing stabilizing measures when the comparingof step (A) is affirmative, further including the step of: observing thereaction of the vehicle to changes in speed, and adjusting the firstthreshold value or a second threshold value of the level of the centerof gravity of the vehicle.
 2. Method according to claim 1, wherein thecorrelating variable is the transversal acceleration in one point whichcorresponds approximately to the location of the vehicle's center ofgravity in an unloaded vehicle.
 3. Method according to claim 1, thecorrelating variable is a function resulting from the vehicle'stransversal acceleration and the time derivative.
 4. Method according toclaim 1, wherein the stabilizing measures are interrupted if the firstthreshold value or the second, lower threshold value are not exceeded.5. Method according to claim 2, wherein the step of determining thetransversal acceleration of the motor vehicle is conducted on the basisof the signals generated by the wheel sensors.
 6. Method according toclaim 1, wherein the transversal acceleration of the motor vehicle isdetermined by a steering angle signal and signals of the wheel sensors.7. Method according to claim 1, wherein said stabilizing measureincludes activating a brake on the front axle.
 8. Method according toclaim 1, wherein said stabilizing measure includes activating thevehicle suspension.
 9. Method according to claim 1, wherein saidstabilizing measure includes activating a measure for preventing theside roll with higher priority over activating a measure for preventingexcessive understeering.
 10. Method for increasing the lateral tiltstability of a vehicle with at least two axles and at least two tracksin which measures are taken for preventing the side roll, comprising thesteps of: A) comparing a variable correlating with the transversalacceleration of the vehicle against a first threshold value wherein thethreshold value represents, at least at the beginning of the travel, thelowest value which leads to tilting risk in case of any admissible loadof the vehicle, B) introducing stabilizing measures when the comparingof step (A) is affirmative, wherein said stabilizing measure includesactivating a brake on the front axle.
 11. Method for increasing thelateral tilt stability of a vehicle with at least two axles and at leasttwo tracks in which measures are taken for preventing the side roll,comprising the steps of: A) comparing a variable correlating with thetransversal acceleration of the vehicle against a first threshold valuewherein the threshold value represents, at least at the beginning of thetravel, the lowest value which leads to tilting risk in case of anyadmissible load of the vehicle, B) introducing stabilizing measures whenthe comparing of step (A) is affirmative, wherein said stabilizingmeasure includes activating a measure for preventing the side roll withhigher priority over activating a measure for preventing excessiveundersteering.